Rigid body simulation always has a visible collision space
This seems to be caused by the Subsurf modifier on the Suzanne.

Frame 124 with the Subsurf modifier:



Frame 124 with the Subsurf applied:



In addition the Backdrop had a collision margin of 4cm, in the images above it is set to 0.

Comparison of frame 124 with the collision margin of the BackDrop set to 4cm and 0m:

 

I also set the Collision shape of the Suzanne to Convex Hull and disabled Collision margin.